According to the Council of Trent:
The Mass is a true and proper sacrifice which is offered to
God.
By the words, "Do this in commemoration of me" (Luke 22:19; I
Corinthians 11:24), Christ made the apostles priests. Moreover, He
decreed that they and other priests should offer His Body and
Blood.
The Sacrifice of the Mass is not merely an offering of praise and
thanksgiving, or simply a memorial of the sacrifice on the Cross.
It is a propitiatory sacrifice which is offered for the living and
dead, for the remission of sins and punishment due to sin, as
satisfaction for sin and for other necessities.
The Sacrifice of the Mass in no way detracts from the sacrifice
which Christ offered on the Cross (Council of Trent, Session XXII,
September 17, 1562).
http://www.therealpresence.org/eucharst/link/e-litur.html
